Recently, UV-rays have been recognized to give harmful influences on the skin and sunscreen cosmetics for preventing UV-rays have been increasingly demanded. In addition, also in makeup cosmetics for helping the skin appear uniform and beautiful, enhancing the UV-shielding effect has been demanded. To satisfy such demands, cosmetics containing a UV-scattering agent such as fine particulate titanium dioxide and fine particulate zinc oxide and an organic UV absorber have been developed; however, these cosmetics are required to be further enhanced in a UV-shielding effect. To enhance a UV-shielding effect, generally, a UV-scattering agent or a UV absorber is added in a large amount. However, if a UV-scattering agent such as fine particulate titanium dioxide is blended in a large amount in a cosmetic, it results in grainy on the skin and a cosmetic forms a thick film to decrease transparency. In addition, a large content of UV absorber has a safety problem. If a person of a sensitive skin uses such a cosmetic, the person may develop a skin trouble such as rash. Actually, the content is limited by regulations, and thus, a large amount of argent cannot be blended. For these reasons, development of a UV-scattering agent, more specifically, titanium dioxide having high UV-shielding performance, in other words, titanium dioxide shielding UV rays in a small content, has been strongly desired.
On the other hand, fine particulate titanium dioxide has a satisfactory shielding effect against the UVB region (280 to 320 nm) and is blended in order to improve SPF (Sun Protection Factor) serving as an index for UVB shielding property; however its effect of shielding UV rays in the UVA region (320 to 400 nm) is insufficient. Therefore, in the country, fine particulate zinc oxide is generally used as a UVA shielding agent. However, in Europe, zinc oxide is not approved as a UV-shielding agent for cosmetics. In addition, if zinc oxide is blended in a large concentration, ions elute and irritate skin. This is a matter of concern. For these reasons, development of titanium dioxide having high UVA shielding effect has been increasingly demanded.
Absorption or scatteration of light by titanium dioxide brings the UV-shielding performance of fine particulate titanium dioxide. The shielding effect against the UVB region is due to absorption of light by a band gap; however, the shielding effect against the UVA region is due to scattering and there is a particle diameter at which maximum shielding performance can be obtained. With respect to the light scattering of titanium dioxide and particle-diameter dependency of UV protection effect, a computational approach based on the Mie theory (P. Stamatakis et al., J. Coatings Tech, 62 (10), 95 (1990)) is made in Japanese Patent Laid-Open No. 9-202722 (Patent Literature 1). According to the results, in the case of light of 300 nm in wavelength, a maximum shield effect is obtained at a particle diameter of 30 to 60 nm; whereas, in the case of light of 350 nm in wavelength, the most suitable effect is obtained at a particle diameter of 80 nm and in the case of light of 400 nm in wavelength, the most suitable effect is obtained at a particles diameter of 120 nm.
In view of this, the present inventors proposed, in Japanese Patent Laid-Open No. 11-322337 (Patent Literature 2), butterfly-shaped rutile-type titanium dioxide formed of aggregated and/or bound needle shaped fine particles, which have an apparent average major axial length (herein, the term “apparent” is used in order to show that the length is a measured value of the aggregated particles; the same shall apply hereinafter) of 0.1 μm (100 nm) or more. The butterfly-shaped titanium dioxide has more excellent UV-shielding performance in the UVA region than a conventional UV-shielding titanium dioxide; however, the performance is not satisfactory. The present inventors further proposed, in Japanese Patent Laid-Open No. 2010-173863 (Patent Literature 3), titanium dioxide having a particulate form configured such that the major axial planes of individual rod-shaped particles (each having a minor axis diameter of 3 to 10 nm) are oriented and aggregated in the minor axial direction. The titanium dioxide is cocoon-shaped rutile-type titanium dioxide, in which an apparent average major axial length of particles oriented and aggregated is 80 to 300 nm; an apparent average minor axial length of particles oriented and aggregated is 30 to 150 nm; an apparent average axial ratio (represented by a value of apparent average major axial length/apparent average minor axial length) is 1.1 to 4; both ends of the major axis of oriented and aggregated particles have a spherical shape or elliptical shape; and a specific surface area is 120-180 m2/g. The cocoon-shaped rutile-type titanium dioxide has more excellent UV-shielding performance in the UVB region than conventional UV-shielding titanium dioxide; however UV-shielding performance in the UVA region was not satisfactory.